1. Field of the Invention
Our invention relates generally to reliable wireless communications between two points in a wireless network. More particularly, our invention relates to methods of providing reliable communications between mobile wireless nodes or subsets positioned at two points in a wireless network, wherein no two nodes between these two points can directly reliably communicate.
2. Description of the Background
A basic necessity for two wireless nodes wishing to communicate in a wireless communications network is for these nodes to overcome the loss and variability of the propagation environment between them to establish reliable communications. This necessity holds true whether the two nodes are both mobile (as seen in military applications and emerging mobile, multi-hop wireless networks) or whether one node is static and the other node is mobile (as seen in commercial applications where the mobile node only communicates with a “base-station”).
One common solution to provide reliable communications is for the transmitter to radiate sufficient or increased power to overcome the propagation environment. The problem with this solution is that it is not always possible to increase transmit power. This holds true for military applications where it is not desirable to increase power because it can divulge a mobile transmitter's position. This solution is also not viable for commercial applications where (1) the power levels of the mobile nodes are decreasing as the units continue to get smaller, and (2) increased power comes at the price of reduced battery life.
Another solution is to add additional base stations and/or repeaters to the wireless network in order to minimize the maximum distances between transmitters and receivers. One problem with these solutions is that it is costly to deploy additional equipment. Another problem is that these solutions are not viable for fast deployable, wireless infrastructure networks, where the networks are dynamically formed and move, as have been seen in the military sector and are emerging in the civilian/commercial sector (further described below as “multi-hop” networks).
A third solution is the emerging mobile, multi-hop wireless network (“multi-hop” networks). In these networks, each mobile wireless node not only acts like a source of data, but also as a router. The basic assumption in these networks is that wireless nodes within the network are willing to forward data for other nodes in the network that are not able to establish direct reliable communications. Multi-hop networks are applicable to military applications. They are also applicable to users wishing to establish “on-demand” networks (such as for rescue missions in times of natural disaster, law enforcement, or business and educational use, etc.), and are applicable to cellular networks where a mobile node may not be able to establish direct communications with a base-station for access to wire-based networks.
Advantageously, multi-hop networks do not require the need for additional static hardware. They also solve the problem regarding increased transmission power, assuming there are enough mobile nodes in close proximity to establish a reliable path. However, this latter assumption may not hold true. For example, under military and emergency applications, the number of mobile nodes may be small and may not be in close proximity. In addition, mobile nodes in general are often turned off when not in use, reducing the number of available units to repeat data. Lastly, mobile nodes expected to be left on for a long duration may have low transmit power to ensure prolonged battery life. Hence, while multi-hop networks present a solution for two distant wireless nodes to establish communications through a multi-hop path, it may not be possible to establish one or more hops along this path without requiring increased transmit power.
Existing technologies allow wireless nodes to form reception and transmission groups to overcome the loss of a propagation environment however, these groups are based on static, not mobile nodes. Code Division Multiple Access (CDMA), which is based on spread spectrum technology, provides a soft hand-off feature that allows a single mobile node to simultaneously transmit and receive signals with more than one base station. The mobile node is capable of coherently combining the signals from the multiple base stations in order to achieve a signal to noise ratio (SNR) that is higher than the SNR from any one base station. This is accomplished through the use of a Rake receiver that introduces the appropriate delay for each received signal and then combines them. Similarly, multiple base stations are able to receive the same signal from a mobile node, each base station demodulating the signal and sending its corresponding information to a collection point, which then selects the best signal on a frame by frame basis.
Similarly, under the existing antenna and adaptive-antenna array technologies, the antennas are physically separated in space from each other and have independent receivers. These antenna systems are capable of receiving separate signals and coherently combining these signals at a central location.
Hence, these current systems/technologies allow multiple stationary units to form dynamic groups that can simultaneously receive and coherently combine a signal transmitted by a single mobile node thereby overcoming a “lossy” propagation environment. Similarly, a single mobile node can receive and coherently combine a signal simultaneously transmitted by the group of stationary units.